Here's a summary of some possibilities:
You could all invest in a simple note (loan) or consider investing in a note that converts into equity in the next round of equity that the company raises. Typically, these notes pay a reasonable rate of interest and convert at a 20% discount to the price of the next round. Presumably you are all investing in the company because you believe in its future and want to expose yourselves to the upside so the convertible note may be a better instrument.
As mentioned, it depends in part on what business form you have, if any, although you still haven't clarified that.
I'm not sure I would consider a loan to be an investment, after all, you're just expecting to get your money back in the future. If you want to get the benefits of your investment it has to be equity. If everyone else doesn't make an additional investment then everyone's percentage of ownership will change.
Sounds like you have a legal entity in place (an s-corp, llc, c-corp, in the US for example).
Assuming everyone owns the equal amount of common shares and everyone will invest the equal amount, issue new shares at the same price as the original shares (eg $1.00) to keep the relative ownerships consistent.
Loan, definitely. Just document it with a promissory note.